Field of the Invention
The invention relates to a device for NO.sub.x reduction in the exhaust gas of an internal combustion engine operated with excess air. The engine is controlled with a control unit. The device includes a reduction catalytic converter that works according to the so-called SCR principle in the exhaust system. A reducing agent metering system has a reducing agent injection valve upstream of the reduction catalytic converter in the exhaust system. The reducing agent is metered in on the basis of catalytic converter operating parameters and the exhaust gas composition.
The so-called selective catalytic reduction (SCR) method is known for reducing the NO.sub.x content in the exhaust gas of an internal combustion engine operated with excess air. A reducing agent is thereby injected into the exhaust gas at a point upstream of a catalytic converter (in terms of the exhaust gas flow direction), and the NO.sub.x contained in the exhaust gas is thus reduced in a reduction catalytic converter to form nitrogen gas N.sub.2. In this regard, reference may be made to the publication by Schoppe et al., "Ein geregeltes Abgasnachbehandlungssystem zur Erfullung zukunftiger Emissionsgrenzwerte bei PKW-Dieselmotoren" [A controlled exhaust gas post-treatment system for meeting future emission restrictions in automobile diesel engines], 17th International Wiener Motorensymposium, 1996, Vol. 1. Ammonia can serve as the reducing agent, although for reasons of convenience it is customary to use an aqueous urea solution or powdered urea. It is also known to employ fuel or derivatives as the reducing agent.
In the known SCR system, the target quantity for the reducing agent metering is calculated continuously by the control unit of the internal combustion engine. For this, the control unit needs the instantaneous reducing agent demand. This is calculated from operating parameters of the internal combustion engine, such as mass of air, operating temperature or load. Together with the catalytic converter temperature, these values form the basis for metering the reducing agent. NO.sub.x reduction using the SCR system allows reduction rates in excess of 60%, but presupposes the determination of individual emission characteristics for the internal combustion engines, which have to be stored in a control unit in order to control the metering.
The NO.sub.x reduction therefore requires significant equipment outlay with extensive data interchange between the control unit of the internal combustion engine and the reducing agent metering system. It has to date been possible only with great outlay to retrofit vehicles which are already being used and have internal combustion engines operated with excess air, with an exhaust gas post-treatment system based on the SCR principle, or to equip internal combustion engines originally developed without NO.sub.x reducing exhaust gas treatment with SCR catalysts.